The disclosure relates generally to additive manufacturing, and more particularly, to a method of controlling microstructure of a selected range of layers of an object during additive manufacture based on the sum of: a melting time, an idle time and a recoating time, of each layer in the range, while maintaining other operation parameters constant.
The pace of change and improvement in the realms of power generation, aviation, and other fields has accompanied extensive research for manufacturing objects used in these fields. Conventional manufacture of metallic, plastic or ceramic composite objects generally includes milling or cutting away regions from a slab of material before treating and modifying the cut material to yield a part, which may have been simulated using computer models, e.g., in drafting software. Manufactured objects which may be formed from metal can include, e.g., airfoil components for installation in a turbomachine such as an aircraft engine or power generation system.
Additive manufacturing (AM) includes a wide variety of processes of producing an object through the successive layering of material rather than the removal of material. As such, additive manufacturing can create complex geometries without the use of any sort of tools, molds or fixtures, and with little or no waste material. Instead of machining objects from solid billets of material, much of which is cut away and discarded, the only material used in additive manufacturing is what is required to shape the object.
Additive manufacturing techniques typically include taking a three-dimensional computer aided design (CAD) file of the object to be formed, electronically slicing the object into layers, e.g., 18-102 micrometers thick, and creating a file with a two-dimensional image of each layer, including vectors, images or coordinates. The file may then be loaded into a preparation software system that interprets the file such that the object can be built by different types of additive manufacturing systems. In 3D printing, rapid prototyping (RP), and direct digital manufacturing (DDM) forms of additive manufacturing, material layers are selectively dispensed, sintered, formed, deposited, etc., to create the object.
In metal powder additive manufacturing techniques, such as direct metal laser melting (DMLM) (also referred to as selective laser melting (SLM)), metal powder layers are sequentially melted together to form the object. More specifically, fine metal powder layers are sequentially melted after being uniformly distributed using an applicator on a metal powder bed. Each applicator includes an applicator element in the form of a lip, brush, blade or roller made of metal, plastic, ceramic, carbon fibers or rubber that spreads the metal powder evenly over the build platform. The metal powder bed can be moved in a vertical axis. The process takes place in a processing chamber having a precisely controlled atmosphere. Once each layer is created, each two-dimensional slice of the object geometry can be fused by selectively melting the metal powder. The melting may be performed by a high power melting beam, such as an ytterbium fiber laser of 50 W to 2000 W output power, to fully weld (melt) the metal powder to form a solid metal. The melting beam moves in the X-Y direction using scanning mirrors, and has an intensity sufficient to fully weld (melt) the metal powder to form a solid metal. The metal powder bed may be lowered for each subsequent two-dimensional layer, and the process repeats until the object is completely formed. In order to create certain objects faster, some metal additive manufacturing systems employ more than one high power laser that work together to form an object or objects.
One challenge with certain additive manufacturing processes, such as DMLM, is that the objects have different microstructures compared to conventionally cast material of the same alloy. The different microstructures are caused by the high energy beam and un-melted material interaction leading to high cooling rates and very fast solidification during these additive manufacturing processes. The issue can be observed between additively manufactured objects versus conventionally cast materials, and can also be observed within individual objects where using the same additive manufacturing operation parameters across an entire object can lead to inhomogeneous microstructure and material properties within the object. In many cases, a heat treatment must be carried out after additive manufacturing in order to adjust the microstructure of the part and to reduce/eliminate residual stresses. However, the vast differences in microstructure cannot always be homogenized by a heat treatment and result in inhomogeneous material properties.
Current additive manufacturing processes attempt to address the situation in a number of ways. One approach attempts to create a largely homogeneous temperature profile. This approach may enable beam melting only when a temperature is below a threshold, or may control beam characteristics such as scanning speed, size of focal point, laser pulse frequency, laser pulse duration and/or laser power, to achieve the homogeneous temperature profile. Alternatively, other approaches may call for a shorter than maximum scan line, and/or provide a time sink before and/or after a scan line, so the time period between adjacent scan lines is constant throughout the article. This time sink however does not fully address inhomogeneous material vertically in an object. Another approach adjusts a traveling speed of the beam along each scan line as a function of the length of the line, e.g., decreasing speed with increasing length of line, to reduce hot spots. Each of these approaches require a complex control algorithm that requires temperature monitoring and real-time adjusting of a number of operation parameters within each layer and across layers.